1. Field of the Invention
The present invention relates to a semiconductor laser apparatus for use as a light source for pumping a solid-state laser, a fiber laser, a fiber amplifier or the like, or for use in various laser instrumentation.
2. Description of the Related Art
In realizing a higher power semiconductor laser, it is important to inhibit catastrophic optical mirror damage at the light-emitting end face of the laser. For the purpose of a higher power semiconductor laser, the present applicant has proposed a semiconductor laser provided with a carrier blocking layer having a bandgap larger than an active layer and a small thickness on each of the opposite sides of the active layer, thereby offering widened design freedom as to the thickness of a waveguide layer and the bandgap of a cladding layer which are formed outside each carrier blocking layer (refer to International Publication No. WO93/16513).
In such a structure, carriers injected into the active layer can efficiently be confined by the carrier blocking layers, while laser light generated at the active layer passes through each thin carrier blocking layer and is propagated along an optical waveguide defined mainly by the waveguide layers and cladding layers. By designing the optical waveguide so as to confine light existing in the waveguide layers to a greater extent, the intensity of light in the active layer lowers and, hence, an optical output power at which the catastrophic optical mirror damage (COMD) occurs on the light-emitting facet can be raised, thus leading to a higher power operation.
Japanese Unexamined Patent Publication JP-A 10-303500 (1998) describes a structure such as to enhance confinement of light in the waveguide layers in order to lower the intensity of light existing in the active layer formed in the separate-confinement structure thereby raising an optical output power at which the COMD occurs on the light-emitting facet.
In the case of such enhanced confinement of light in the waveguide layers as in JP-A 10-303500, guided modes which can be propagated along the optical waveguide include not only a fundamental mode but also higher-order modes.
From the standpoint of the lasing conditions, the intensity of light in the active layer creating a gain reaches the highest value in the fundamental mode, and in addition the free carrier absorption and the substrate radiation loss become smaller in the fundamental mode. This means that lasing is likely to occur in the fundamental mode.
However, the present inventors have discovered the fact that the optical waveguide can propagate a higher-order mode as well as the fundamental mode in which lasing occurs and, hence a undesirable wavelength separation occurs in the lasing spectrum of laser light. Such a wavelength separation causes a full width at half maximum to increase and inhibits the oscillation wavelength to continuously vary even when the temperature or the injection current is continuously varied. For this reason, use of such a semiconductor laser as a light source for pumping a solid-state laser, fiber laser, fiber amplifier or the like results in a decrease in pumping efficiency and an unstable output. Thus, problems will be raised in applications.
It is an object of the invention to provide a semiconductor laser apparatus which is capable of a stabilized high power operation by inhibiting wavelength separation caused by a higher-order mode in an epitaxial direction.
The invention provides a semiconductor laser apparatus comprising an active layers, n-type and p-type optical waveguide layers provided on both sides of the active layer, the n-type and p-type optical waveguide layers each having a bandgap larger than that of the active layer, and n-type and p-type cladding layers disposed so that the active layer and the optical waveguide layers are sandwiched therebetween, the n-type and p-type cladding layers each having a bandgap larger than that of each of the waveguide layers,
wherein a condition represented by the following formula (A) is satisfied:
nc1xe2x89xa6Re(ne1) xe2x80x83xe2x80x83(A) 
wherein nc1 represents a smaller one of refractive indices of the n-type and p-type cladding layers, Re(ne1) represents a real part of an effective refractive index of a first-order guided mode of an optical waveguide including the optical waveguide layers and the cladding layers, and
wherein compositions of the n-type and p-type cladding layers are made asymmetrical, whereby no higher-order waveguide mode exists in an epitaxial direction.
According to the invention, by setting the real part Re(ne1) of the effective refractive index of the first-order guided mode to be equal to or larger than that of the smaller one of the refractive indices of the n-type and p-type cladding layers the intensity of light existing in the active layer can be lowered, thereby raising an optical power at which the catastrophic optical mirror damage occurs on the light-emitting facet. Thus, a higher power operation can be realized.
Furthermore, according to the invention, by making the compositions of the n-type and p-type cladding layers asymmetrical, a higher-order waveguide mode in the thickness direction is avoided. Accordingly only a fundamental mode can be propagated with low loss and wavelength separation in oscillation spectrum can be prevented.
In the invention it is preferable that an optical waveguide including the optical waveguide layers and the cladding layers satisfies a condition represented by the following formula (1):                                           t            λ                    ⁢                                                    nw                2                            -                              nc1                2                                                    ≧                  1          2                                    (        1        )            
wherein xcex represents a laser wavelength in a vacuum, t represents a thickness between the n-type and p-type cladding layers, nw represents a refractive index of the n-type and p-type optical waveguide layers, nc1 represents the smaller one of the refractive indices of the n-type and p-type cladding layers, and nc2 represents a larger one of the refractive indices of the n-type and p-type cladding layer.
According to the invention, the condition represented by the formula (1) is satisfied, so that the intensity of light existing in the active layer can be lowered, thereby raising an optical output at which the COMD occurs on the light-emitting facet. Thus, a higher power operation can be realized.
In the invention it is preferable that a condition represented by the following formula (2) is satisfied:
Re(ne1) less than nc2xe2x89xa6ne0 xe2x80x83xe2x80x83(2) 
wherein ne0 represents an effective index of a fundamental guided mode in the optical waveguide, and Re(ne1) represents the real part of the effective index of a first-order guided mode.
According to the invention, the condition represented by the formula (2) is satisfied, so that the fundamental guided mode can be propagated in middle layers and the first-order guided mode leaks into the cladding layer having the refractive index nc2. As a result only the fundamental guided mode can be propagated with low loss and wavelength separation in oscillation spectrum can be prevented.
The principle of the invention is as follows. FIG. 4 is a sectional view illustrating a typical structure of a semiconductor laser apparatus. On a substrate 111 of n-type GaAs are sequentially formed using metalorganic vapor phase epitaxy (MOVPE) or a like process an n-type cladding layer 112 (AlGaAs, Al context x=0.20, thickness t=1.2 xcexcm), an n-type optical waveguide layer 113 (GaAs, t=0.49 xcexcm), an n-type carrier blocking layer 114 (AlGaAs, x=0.40, t=0.03 xcexcm), an active layer 115 (composed of an In0.28Ga0.82As quantum well layer and GaAs barrier layers), a p-type carrier blocking layer 116 (AlGaAs, x=0.40, t=0.03 xcexcm), a p-type optical waveguide layer 117 (GaAs, t=0.49 xcexcm), and a p-type cladding layer 118 (AlGaAs, x=0.20, t=1.2 xcexcm). On the p-type cladding layer 118 is further formed a p-type cap layer 120 (GaAs), in which a pair of n-type current blocking layers 119 (GaAs) are buried to form a current injection region 101 (width=100 xcexcm).
A resonator extending in a direction perpendicular to the plane of the drawing is 2.2 mm long and has a light-emitting facet formed with an optical coating having a reflectance of 2% and an opposite facet with an optical coating having a reflectance of 96% after cleavage. Electrodes (not shown) are formed on the underside of the substrate 111 and the top of the p-type cap layer 120, respectively.
FIG. 5 is a graph showing a variation with temperature in oscillation spectrum of the semiconductor laser apparatus 100 of FIG. 4, in which graph the vertical axis represents peak wavelength (nm) and the horizontal axis represents case temperature (xc2x0 C.), and the operation current Iop was 2 (A). As seen from the graph, the peak wavelength varied linearly with temperature at a rate of change of about 0.25 nm/xc2x0 C., and the peak wavelength varied stepwise at about 25xc2x0 C. and about 50xc2x0 C., which proves that a wavelength separation occurred.
FIG. 6 is a graph showing an oscillation spectrum of the semiconductor laser apparatus 100 of FIG. 4, in which graph the vertical axis represents spectral band intensity (dBm), and the horizontal axis represents wavelength (nm). This graph shows a wavelength separation that occurred when the case temperature was 50xc2x0 C., in which two peaks appeared having respective wavelengths of 984.7 nm and 987.9 nm with a separation spacing of about 3.2 nm.
In analyzing an optical waveguide of a multilayered slab structure such as a semiconductor laser apparatus, the solution to a Maxwell""s equation can be found by a stepwise partitioning method (refer to Kenji Kono, xe2x80x9cFundamental and Applications of Optical Coupling Systems for Optical Devicesxe2x80x9d, 1991). From the analysis, the semiconductor laser apparatus 100 shown in FIG. 4 is found to allow propagation of the fundamental (zero-order) mode and the first-order mode.
FIG. 7 is a graph showing the refractive index distribution and effective refractive index and mode profile of each mode of an optical waveguide in a semiconductor laser apparatus 100 of FIG. 4. In the graph, the vertical axis represents refractive index, and the horizontal axis represents the location (xcexcm) from the interface between the n-type cladding layer 112 and the n-type optical waveguide layer 113, the interface being the point or origin, an upper location from the interface assuming a positive value.
In the refractive index distribution, the refractive indices of the n-type and p-type cladding layers 112 and 118 are low, while that of the central portion sandwiched between these cladding layers is high, thus forming an optical waveguide. From the results of the analysis, the profile of the fundamental mode is found to be a single-peak curve having slight dents adjacent the carrier blocking layers 114 and 116, while the profile of the first-order mode found to be a twin-peak curve. The effective indices ne0 and ne1 of the fundamental mode and the first-order mode are defined by eigenvalues of a Maxwell""s equation and can be found by numerical calculations. In the case of the semiconductor laser apparatus 100 shown in FIG. 4, ne0=3.509, and ne1=3.478.
The mechanism of an occurrence of wavelength separation is as follows. Where the modes which can be propagated along an optical waveguide are the fundamental mode and the first-order mode, faint mode conversions from the fundamental mode to the first-order mode and vice versa occur at the opposite facets of a semiconductor laser. Since there is a difference in effective refractive index between modes of different mode numbers, the optical resonator length of the fundamental mode differs from that of the first-order mode. It can be conceived that a sort of double resonator effect occurs through such mode conversions.
The resonance condition of a laser resonator is considered here. The resonance condition of a fundamental mode is given by the following formula (11):
m0xc2x7xcexm0=2xc2x7Lxc2x7ne0 xe2x80x83xe2x80x83(11) 
wherein xcexm0 represents the wavelength of light; L represents a resonator length, ne0 represents the effective refractive index of the fundamental mode, and m0 represents a number of loops of a standing wave.
As the number of loops of a standing wave increases or decreases one by one, the lasing wavelength varies discontinuously, and taking the wavelength dependence of the effective refractive index into consideration, a wavelength spacing xcex94xcex0 is given by the following formula (12):                               Δ          ⁢                      xe2x80x83                    ⁢          λ          ⁢                      xe2x80x83                    ⁢          0                =                              λ            2                                2            ·                          L              ⁡                              (                                  ne0                  -                                      λ                    ·                                                                  ∂                        ne0                                                                    ∂                        λ                                                                                            )                                                                        (        12        )            
On the other hand, the resonance condition of the first-order mode is given by the following formula (13):
m1xc2x7xcexm1=2xc2x7Lxc2x7ne1 xe2x80x83xe2x80x83(13) 
wherein xcexm1 represents the wavelength of light, L represents the resonator length, ne1 represents the effective refractive index of the first-order mode, and m1 represents the number of loops of a standing wave.
Taking the wavelength dependence of the effective refractive index into consideration, the wavelength spacing xcex94xcex1 in the first-order mode is given by the following formula (14):                               Δ          ⁢                      xe2x80x83                    ⁢          λ          ⁢                      xe2x80x83                    ⁢          1                =                              λ            2                                2            ·                          L              ⁡                              (                                  ne1                  -                                      λ                    ·                                                                  ∂                        ne1                                                                    ∂                        λ                                                                                            )                                                                        (        14        )            
Where mode conversions from the fundamental mode to the first-order mode and vice versa occur at the opposite end faces of the semiconductor laser, a loss is considered to be minimized at a wavelength which satisfies both the resonance condition (11) of the fundamental mode and the resonance condition (13) of the first-order mode. The wavelength spacing xcex94xcex01 in this case, which corresponds to a wavelength separation spacing, is given by the following formula (15):                               Δ          ⁢                      xe2x80x83                    ⁢          λ          ⁢                      xe2x80x83                    ⁢          01                =                              λ            2                                              2              ·              L                        ⁢                          {                              (                                  ne0                  -                                      λ                    ·                                                                  ∂                        ne0                                                                    ∂                        λ                                                                              -                  ne1                  -                                      λ                    ·                                                                  ∂                        ne1                                                                    ∂                        λ                                                                                            )                            }                                                          (        15        )            
Here, such considerations are applied to the semiconductor laser apparatus 100 as shown in FIG. 4. Since the effective refractive indices of the fundamental mode and first-order mode are ne0=3.509, and ne1=3.478, respectively, and the relation between the refractive index of an AlGaAs semiconductor and the Al content x is known, the effective Al contents for the fundamental mode and the first-order mode are found to be 0.048 and 0.10, respectively. Further, the wavelength dependences (differential terms) of the refractive indices at these effective Al contents are found to be xe2x88x920.49 (/xcexcm) in the fundamental mode and xe2x88x920.45 (/xcexcm) in the first-order mode.
When these values and L=2.2 mm and xcex=984.7 nm are substituted into the formula (15), the wavelength separation spacing is found to be xcex94xcex01=3.1 nm, which substantially agrees to the experimental value, or about 3.2 nm, obtained in FIG. 6. Thus, a wavelength separation as shown in FIG. 6 is found to derive from the double resonator effect.
While the foregoing description has been made of an illustrative construction including carrier blocking layers like the semiconductor laser apparatus 100 shown in FIG. 4, it is possible that a wavelength separation in oscillation spectrum as described above occurs in semiconductor lasers having optical waveguides which can propagate a higher-order mode, in general.
To solve such a problem, there is provided a semiconductor laser in which the intensity of light existing in the active layer thereof is lowered and, hence, an optical output at which the COMD on the light-emitting facet of the laser occurs is raised by virtue of the provision of an optical waveguide including optical waveguide layers and cladding layers which satisfies a condition represented by the formula (1):                                           t            λ                    ⁢                                                    nw                2                            -                              nc1                2                                                    ≧                  1          2                                    (        1        )            
where xcex represents the wavelength of laser light in a vacuum, t represents the thickness between two optical cladding layers, nw represents a refractive index of the optical waveguide layers, nc1 represents the smaller one of the refractive indices of the two optical cladding layers, and nc2 represents the larger one of the refractive indices of the cladding layers, and which optical waveguide further satisfies the condition represented by the formula (2):
Re(ne1) less than nc2xe2x89xa6ne0 xe2x80x83xe2x80x83(2) 
wherein ne0 represents the effective refractive index of a fundamental guided mode, and Re(ne1) represents the real part of the effective refractor index of the first-order guided mode. In this semiconductor laser, the fundamental guided mode can propagate in the middle layers, while the first-order guided mode leaks into the cladding layer having the refractive index nc2. As a result, only the fundamental guided mode can propagate with low loss thereby inhibiting the occurrence of wavelength separation in oscillation spectrum.
In the semiconductor laser apparatus 100 shown in FIG. 4, for example, the effective refractive index ne1 of the first-order guided mode is 3.478, the refractive index nc2 of the cladding layer having the larger refractive index is 3.42, and the effective index ne0 of the fundamental mode is 3.509, and these values do not satisfy the formula (2). Thus, the semiconductor laser apparatus 100 permits propagation of the first-order mode undesirably.
Thus, only when both the conditions of the formulae (1) and (2) are satisfied, it is possible to realize stabilized lasing of only the fundamental guided mode with propagation of the first-order mode inhibited, while lowering the intensity of light existing in the active layer to ensure a higher power operation. Further, since the light intensity of the fundamental mode is distributed widely and outwardly beyond the active layer 515 as shown in FIG. 2, the intensity of light existing in the active layer 515 further lowers, so that an optical output at which the COMD on the light-emitting facet can be raised, thereby enabling a further higher power operation.
In the invention it is preferable that a carrier blocking layer is provided between the active layer and each of the optical waveguide layers, the carrier blocking layer having a bandgap larger than those of the active layer and the optical waveguide layer.
The provision of the carrier blocking layer having a larger bandgap between the active layer and each of the optical waveguide layers allows confinement of carriers in the active layer and confinement of light in the optical waveguide layers to function independently of each other. This improves the temperature characteristics of the semiconductor laser thereby enabling a higher power operation.
The invention becomes more advantageous when applied to a semiconductor laser having a refractive index distribution structure in the layers for controlling the transverse mode of laser light.
In some applications, it is desirable that each of the oscillation mode configurations of both the vertical mode and the transverse mode in the semiconductor laser of the invention should be a single Gaussian profile. This is because a semiconductor laser generating a single mode can emit light of a higher density thereby enabling easy coupling to an optical fiber.
To this end, an effective difference in refractive index is also provided in the transverse direction by utilizing an SAS (self-aligned structure) in which layers having different refractive indices are formed on opposite sides of a stripe, a ridge structure in which both sides of a stripe are etched, or interlayer diffusion of constitutional atoms achieved by ion implantation, thereby to form an optical confinement structure.
In the design of a refractive index distribution, it is assumed that confinement of the vertical mode and that of the transverse mode, generally, are in an independent relationship to each other. Actually, however, the two influence each other. For this reason, where the vertical mode confinement is designed to admit a higher-order mode, there may occur an undesirable wavelength separation or a transverse mode disturbance.
Thus, by inhibiting the occurrence of a higher-order vertical mode according to the invention, it is possible to stabilize the transverse mode as well and material good vertical and transverse Gaussian profiles, thereby obtaining satisfactory current vs. output characteristic without a kink until a higher optical output is reached.